NASA Ames scientist holds his breath as rocket comes home.

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The SpaceX Dragon capsule will make history again early Thursday morning when it does something unmanned capsules don't normally do: make it through the atmosphere.

Normally unmanned cargo capsules make a one-way trip to the International Space Station. Astronauts on the station unload them and then fill empty capsules full of dirty clothes and trash and send them to burn up on reentry.

SpaceX wants its capsule back, so late Wednesday morning NASA cleared Dragon to separate from the ISS and start its return to Earth.

The Dragon capsule will be "placed on display" says spokeswoman Kirstin Grantham, though she declined to say where. Future Dragon capsules, however, will be used over and over, much like the space shuttle.

SpaceX, founded by Tesla CEO Elon Musk, hopes to bring the costs of space access down and reusable capsules are a key part of that plan. To bring it home safely, the Dragon is protected by an advanced heat shield developed at Mountain View's NASA Ames.

The Phenolic Impregnated Carbon Ablator, or PICA (pronounced PEEKuh) has only been tested twice before, scientists feel confident their invention will keep the capsule in one piece. The first version of PICA protected the Stardust space probe as it returned to Earth in 2006.

Stardust was the fastest ever man-made object to travel through the atmosphere -- 28,000 miles per hour.

PICA's next test will be protecting the enormous $2.5 billion unmanned Mars Science Laboratory as it enters the Martian atmosphere in August. Mission planners had originally planned to use a different technology but were impressed by PICA's performance.

Photo caption: A test of the PICA-X heat-shield at NASA Ames Research Center. Photo credit: SpaceX